Accompanying the popularization of portable devices such as cellular telephones and notebook-sized personal computers, a small size and lightweight secondary battery having high energy density is required. As a secondary battery to fulfill such characteristics, there is the lithium ion secondary battery, and research and development is being actively performed, and practical use thereof is progressing.
Moreover, in the automotive field, the demand for electric cars that are able to address resource and environmental problems is increasing. Therefore, a battery for driving a motor for electric car use and hybrid car use is required. As a secondary battery which is applicable as a battery for driving a motor, the appearance of a lithium ion secondary battery that is inexpensive, and at the same time, has a large capacitance, and for which cycle characteristics and output characteristics are excellent, is much anticipated.
In a lithium ion secondary battery, as the negative electrode active material, materials in which Li ions are able to be desorbed and inserted such as lithium metal, lithium alloy, metallic oxides, and carbon are used.
As the positive electrode active material, lithium containing complex oxides in which Li ions are able to be desorbed and inserted, such as a lithium-cobalt complex oxide (LiCoO2), and lithium-nickel complex oxide (LiNiO2), are representative.
In such lithium containing complex oxides, LiCoO2 is relatively easy to synthesize, and at the same time, when it is used as a positive electrode active material for a secondary battery, a 4V class high voltage is able to be obtained. Therefore, the secondary battery in which LiCoO2 is used as the positive electrode active material, is anticipated as a secondary battery having a high energy density, and actual practical use thereof is progressing. Furthermore, regarding the lithium ion secondary battery which uses LiCoO2, research and development for obtaining excellent initial capacitance characteristics and cycle characteristics is progressing, and already a variety of results have been obtained.
However, with LiCoO2, because an expensive cobalt compound is used as the main raw material, this becomes a cause for cost increase. Actually, the unit cost per capacitance of a secondary battery in which LiCoO2 is used as the positive electrode active material, is approximately four times more expensive than that of a nickel-hydrogen battery which has already been put to practical use as a secondary battery. Therefore, the applicable usage of the secondary battery in which LiCoO2 is used as the positive electrode active material, is exclusively limited to the portable device field such as cellular telephones and notebook-sized personal computers.
If the positive electrode active material cost for a lithium ion secondary battery can be reduced, and a less expensive lithium ion secondary battery manufactured, it is possible to expand the application from not only small size secondary battery usage for currently popularized portable devices, but also to large size secondary battery usage for electric power storage and electric cars, so that industrially this has enormous significance.
Examples of positive electrode active materials other than LiCoO2 for lithium ion secondary batteries include, lithium-nickel complex oxide (LiNiO2) which uses nickel that is cheaper than cobalt. For the lithium ion secondary battery in which LiNiO2 is used as the positive electrode active material, a higher capacitance than for the lithium ion secondary battery in which LiCoO2 is used as the positive electrode active material, can be expected. Furthermore, since this shows a high battery voltage similarly to the lithium ion secondary battery in which LiCoO2 is used as the positive electrode active material, development is being actively performed.
However, a lithium ion secondary battery in which LiNiO2 is used as a positive electrode active material, has the following defects. That is to say, compared to the lithium ion secondary battery in which LiCoO2 is used as the positive electrode active material, it is inferior in cycle characteristics, and at the same time, in a case of usage or storage under a high temperature environment, the battery performance is relatively easily impaired.
Therefore, it is an object to solve these defects, and various proposals have been made regarding the abovementioned LiNiO2.
For example, in patent document 1 (Japanese Unexamined Patent Publication No. Hei 8-213015), with an object of improving self-discharge characteristics and cycle characteristics of a lithium ion secondary battery, there is proposed lithium containing complex oxides expressed by LixNiaCobMcO2 (where, 0.8≦x≦1.2, 0.01≦a≦0.99, 0.01≦b≦0.99, 0.01≦c≦0.3, and 0.8≦a+b+c≦1.2, and M is at least one kind of element selected from Al, V, Mn, Fe, Cu and Zn).
Moreover, in patent document 2 (Japanese Unexamined Patent Publication No. Hei 8-45509), as a positive electrode active material that is able to maintain excellent battery performance for storage or usage under a high temperature environment, there is proposed lithium containing complex oxides and the like expressed by LiwNixCoBzO2 (where, 0.05≦w≦1.10, 0.5≦x≦0.995, 0.005≦z≦0.2, and x+y+z=1).
The lithium-nickel complex oxides that are proposed in patent documents 1 and 2, both have higher charge capacity and discharge capacity compared to LiCoO2. Furthermore, the cycle characteristics are also improved compared to conventional lithium-nickel complex oxide that is expressed by LiNiO2. However, when manufactured by the conventional manufacturing method, the internal resistance is increased, and the output characteristics are not sufficient.
The causes are, mainly, because the electroconductivity of the positive electrode active material is low, and at the same time, Li ion diffusivity is not sufficient. Hence, at the time of making a battery, in order to ensure sufficient electroconductivity, it is necessary to increase the amount of conductive materials that are mixed with the positive electrode active material. As a result, there is a problem in that the capacity per mass and the capacity per volume for the overall battery become small.
In patent document 3 (Japanese Unexamined Patent Publication No. 2001-52704), it is disclosed that in a complex oxide expressed by the general formula LiwAvQxCoyO2 (where, A is at least one kind or more selected from Ge, Y, Si, Zr, and Ti; Q is at least one kind or more selected from Ni, Mn, Fe, and Al; and w, v, x, y are respectively the range of 0≦w≦1.2, 0.02≦v≦0.125, 0.01≦x≦0.175, 0.01≦x/y≦0.25), by making this mainly a hexagonal and/or a monoclinic crystal structure, and structuring a compatible phase in which two or more kind of phases having similar crystal form and dissimilar lattice constants, are contacted together across a grain boundary, a high power capacity positive electrode material and a secondary battery are provided.
However, synthesis thereof is complicated, and a long time is necessary for the synthesis time. Moreover, it is difficult to manufacture while maintaining the abovementioned structure, and hence the performance thereof is unstable.
In patent document 4 (Japanese Unexamined Patent Publication No. 2000-323123), there is disclosed a positive electrode active material comprising particles of lithium complex oxide which are porous particles consisting primarily of at least one kind or more of elements selected from the group of Co, Ni, and Mn, and lithium, and for which a pore mean diameter by pore distribution measurement using a mercury penetration method is in a range between 0.1 and 1 μm, and the total volume of pores having a diameter between 0.01 and 1 μm, is 0.01 cm3/g or more, characterized in that the particles of the lithium complex oxide are spherical secondary particles, the average particle size of the spherical secondary particles are between 4 to 20 μm, the tap density is 1.8 g/cm3 or more, and the inflection point of the volume decreasing rate by the Cooper plot method is 500 kg/cm2 or more.
However, it is considered that if there are pores with a diameter of 0.1 μm or more, the tap density becomes small, and the contact area with the electrolyte per positive electrode active material unit mass increases, so that an improvement in the load characteristics is possible. However, because the contact area with the electrolyte is increased excessively, charge and discharge of the positive electrode active material is repeated, so that there is concern of deterioration in the life property of the secondary battery.
Moreover, in the Cooper plot method, active material powders are pelletized and pressed, and the particle strength is measured. Therefore, it is considered that rather than the strength of porous and non-porous particles being compared, the filling property and strength of the active material powders are compared. The higher the filling property, even under the same pressure, the pressure per one particle is decreased. Therefore, it is considered that the particle strength cannot be evaluated precisely. Hence, regarding the particles which have a similar tap density, the problem remains in that it is difficult to compare the rupture strength of the particle itself.
Furthermore, use of a hydroxide raw material which has a small tap density, and a double baking process for making the material porous are required. Therefore the production efficiency of the positive electrode active material is decreased, which is not desirable.
[Patent Document 1]
Japanese Unexamined Patent Publication No. Hei 8-213015
[Patent Document 2]
Japanese Unexamined Patent Publication No. Hei 8-45509
[Patent Document 3]
Japanese Unexamined Patent Publication No. 2001-52704
[Patent Document 4]
Japanese Unexamined Patent Publication No. 2000-323123